1. pl Field of the Invention
The present invention relates to a thermal transfer ink sheet used for a thermal printer or a thermal typewriter and, more particularly, to a thermal transfer ink sheet which can be re-used a number of times.
2. Description of the Related Art
A conventional thermal transfer sheet has a simple structure wherein a hot-melt ink layer consisting of a hot-melt binder and a coloring agent is formed on a base film. Therefore, the conventional thermal transfer sheet has the following drawbacks. More specifically, a conventional thermal transfer sheet is a so-called one-time type thermal transfer sheet. In other words, almost no ink layer of a recording medium can be left on a base film upon a single transfer operation, and only a print with density variations can be obtained in a second use and thereafter. Therefore, cost for obtaining a printed record is increased. Since print marks are clearly left on the thermal transfer sheet by only a single printing operation, this poses a problem of confidentiality. Under these circumstances, studies on a reusable thermal transfer sheet have been made.
In a conventional reusable thermal transfer sheet, a thermal transfer material is proposed wherein a heat-resistant resin layer having a microporous film net-like structure is provided on a base film, and hot-melt ink is contained in the pores of the resin layer (Japanese Patent Disclosure (Kokai) No. 55-105579). The transfer material is receiving a lot of attention as a means for controlling a transfer amount of ink in accordance with application energy level of a heating body of a thermal head. However, a porosity is limited due to a required mechanical strength of the porous layer serving as an ink holding body, and hence, an amount of ink filled in pores is limited. For this reason, an ink sheet in which ink of the same volume as that of a porous body is held in the porous body becomes bulky as compared to an ink sheet which does not contain such a large amount of ink. This is disadvantageous in terms of thermal sensitivity. Even if a high-density transferred image is obtained upon application of high energy, an image with good printing quality cannot be obtained. Hot-melt ink heated by a thermal head is not perfectly transferred to a transfer body, thus preventing efficient utilization of ink.
In order to efficiently use ink, another thermal transfer sheet is proposed wherein a hot-melt ink layer is provided on a base film, and a porous film is provided on the ink layer (Japanese Patent Disclosure (Kokai) No. 60-135294). In this transfer sheet, since the thickness of the porous film is set to be smaller than that of the hot-melt ink layer, an amount of ink held and left in a porous body after the sheet is subjected to printing by a thermal head can be small, thus improving utilization efficiency of ink. However, since no ink is filled in advance in the porous body, unnecessary energy is wasted until ink passes through the film, resulting in poor thermal response. In this thermal transfer sheet, transfer media having different contraction stresses are stacked. Therefore, this multi-layered sheet tends to be curled, and travel interference during a printing operation easily occurs. In addition, since a bonding strength between an ink surface and the porous body is not sufficient, the porous body may be broken during the printing operation. In order to increase the number of repetitions, if an ink coating amount is increased, the ink layer is easily peeled from the sheet base. For this reason, the conventional reusable thermal transfer sheet cannot be put into practical use. In addition, a recording sheet is limited, and printing cannot be satisfactorily performed on a normal paper sheet having a surface roughness of 30 to 50 sec (Bekk smoothness).